EP3325280B1 - Device and method for optically encoding an image - Google Patents

Description

STATE OF THE ART Technical area

The present invention relates to a device for optical coding an image-type spatial information and a method for optical coding of such information; the invention applies in particular to the protection against counterfeiting of valuables.

State of the art

Many devices are known for protecting against counterfeit valuable objects such as jewelry, perfume bottles, clothing, bank cards, bank notes.

These devices include, for example, optical security elements which provide variable visual effects depending on the angle of incidence and / or observation, such as diffractive elements, holograms, etc. It is thus possible by means of these optical elements to mark valuables securely by spatial information, for example identification information such as images, alphanumeric characters, etc.

Among the optical security elements, there are also known plasmonic devices comprising metal-dielectric interfaces, the metal layer being structured in the form of arrays to allow the coupling of an incident wave of a given wavelength to a surface plasmon.

The patent application US 20120015118 describes for example a method for controlling the color of a metal, using the excitation of surface plasmons on a metal-dielectric interface when the interface is illuminated by incident optical radiation. This method is applied in particular to the protection against counterfeiting of valuables such as banknotes. More specifically, the aforementioned patent application describes the formation of a set of sub-wavelength metal structures, arranged in a repetitive manner and obtained for example by embossing a metal surface, so as to allow the coupling at a given resonance wavelength of incident lightwaves with surface plasmons. The observed optical effect, strongly dependent on the angle of incidence and / or observation, includes in particular a changing the color of the metal surface, making it possible to create metallic patterns of variable color on valuables for aesthetic purposes and / or protection against counterfeiting.

However, a coupling structure etched in the form of a network requires dimensions of the order of a dozen or so wavelengths to obtain an efficient coupling of an incident light wave with a surface plasmon, the coupling resulting from the collective effect of sub-wavelength structures. The method thus described in the application US 20120015118 is therefore not suited to the formation of miniature marking devices objects to secure, typically less than a few tens of microns, for example for the protection against counterfeiting of small objects. Furthermore, the angular dependence of the visual effect obtained can be a strong constraint for certain applications such as jewelery or jewelery, where one seeks to obtain colors independent of the angle of observation.

WO 2013/039454 A1 discloses an optical coding device of the same kind as that of claim 1, except that the nano-antennas are of Insulator-Metal type.

The publication CUI ET AL: "Plasmonic and metamaterial structures as electromagnetic absorbers", LASER PHOTONICS REV., Vol. 8, no. 4, 2014, pp. 495-520, Wiley Online Library [DOI: 10.1002 / lpor.201400026], describes a structure with plasmonic nano-antennas of MIM (Metal-Dielectric-Metal) type, but not coding an image comprising at least two pixels of different values.

The present description aims to propose an optical coding device allowing the spatial coding of data on elementary meshes whose dimensions can be of the order of the wavelength, notably allowing the realization of miniature marking devices for protection against counterfeiting.

ABSTRACT

• chaque nano-antenne plasmonique est résonante à au moins une longueur d'onde comprise dans ladite première bande spectrale d'observation,
• les nano-antennes plasmoniques sont agencées spatialement sur le support de telle sorte à ce qu'à un pixel de l'image soit associé un sous-ensemble d'une ou plusieurs nano-antenne(s) plasmonique(s) dont la réponse optique selon une polarisation et dans une bande spectrale comprise dans la première bande spectrale d'observation correspond à une valeur dudit pixel, l'ensemble des nano-antennes plasmoniques formant ainsi au moins un premier codage spatial de ladite image dans ladite première bande spectrale d'observation.

According to a first aspect, the present description relates to a device for optical coding of at least one spatial information or "image", which can be decomposed into pixels each defined by a position in the image and at least one value, an image comprising at least two pixels of different values. The coding device is intended to be observed in at least a first observation spectral band and comprises a support and a set of metal-dielectric-metal plasmon nano-antennas formed on said support, such as:

• each plasmonic nano-antenna is resonant at at least one wavelength included in said first spectral band of observation,
• the plasmonic nano-antennas are arranged spatially on the support so that at a pixel of the image is associated a subset of one or more plasmonic nano-antenna (s) whose optical response according to a polarization and in a spectral band included in the first spectral band of observation corresponds to a value of said pixel, the set of plasmonic nano-antennas thus forming at least a first spatial coding of said image in said first spectral band of observation.

In known manner, a plasmonic nano-antenna is resonant at a given wavelength λ _R , called the resonance wavelength, if its length, measured in a given direction, is equal to λ _R / 2np, where n is the refractive index of the dielectric material forming the metal - dielectric - metal structure, p is a non - zero natural integer.

Such a plasmonic nano-antenna has a localized resonance, that is to say that it is able to generate an optical response in a spectral band centered around the resonance wavelength, on an effective section whose surface is of the order of the square of the resonance wavelength.

By "optical response" is meant in the present description an optical response measured in far field, that is to say at a distance greater than twice the wavelength; the optical response can result from a measurement of a reflected luminous flux or the measurement of a flux emitted due to the thermal emission of the nano-antennas.

It is thus possible thanks to an arrangement of a set of plasmonic nano-antennas, to encode spatial information comprising a set of "pixels" (or elementary information elements) each defined by a position and one or more values (s). ): the geometrical parameters (shape, dimensions, orientation) of a plasmonic nano-antenna arranged on the support at a given position are chosen to generate, in a spectral band included in the spectral band of observation, an optical response corresponding to the value of a pixel located at a corresponding position in the image.

Such a coding device can be observed in emission (thermal emission of nano-antennas heated by an external heat source) or in reflection (under illumination with a given incident flow). In the latter case, the resonant absorption of the nano-antennas results in a modification of the spectrum of the wave reflected with respect to the incident wave.

The spectral band of observation can be in a range of wavelengths ranging from UV (300 - 450 nm) to THz (up to 300 microns).

According to one or more exemplary embodiments, the first spatial coding comprises a grayscale coding; the geometrical parameters of the subset of one or more nano-antenna (s) associated with a pixel are chosen to generate an optical response measured according to a polarization and in a given spectral band included in the spectral band is observed, exhibiting a relative intensity ("gray level") varies according to the position and corresponding to the value of the pixel, itself defined as a gray level in a scale of gray levels.

According to one or more exemplary embodiments, the materials of which are formed the metal-dielectric-metal structures constituting the nano-antennas as well as the shape of the nano-antennas being chosen, a variation of the dimensions and / or the orientation of the nano- antennas makes it possible to vary the relative intensity of the optical response in a given spectral band and / or according to a given polarization by degrading the optimal conditions for resonant absorption or emission of the nano-antennas.

According to one or more exemplary embodiments, a subset of plasmonic nano-antennas associated with a pixel comprises plasmonic nano-antennas having different resonance wavelengths for the same polarization; an optical response is then obtained in a spectral band covering all the resonant wavelengths resulting from an additive synthesis of the optical responses of each of the nano-antennas in the case of a transmission observation or we obtain a optical response resulting from a subtractive synthesis of the optical responses of each of the nano-antennas in the case of an observation in reflection. In this case, a grayscale coding can be obtained by adjusting the presence or absence of resonant nano-antennas at different wavelengths.

According to one or more exemplary embodiments, the first spatial coding is a "color coding" of the image; a pixel of the image having at least first and second values defined for example at distinct wavelengths, it is possible to associate with each pixel a subset of nano-antennas having a given polarization, at least a first and a second optical response respectively in a first and a second spectral band in the first observation spectral band, the first and second optical responses corresponding to the first and second pixel values, in order to reproduce the color coding of the 'picture.

According to one or more exemplary embodiments, at least a portion of the plasmonic nano-antennas of the set of plasmonic nano-antennas is resonant according to a first polarization and at least a portion of the plasmonic nano-antennas of the set of nano-antennas plasmonic is resonant according to a second polarization; in these examples, the plasmonic nano-antennas may be arranged spatially on the support so as to form a first spatial coding of a first spatial or image information, observable according to the first polarization, and a second spatial coding of a second spatial information or image, observable according to the second polarization.

According to one or more exemplary embodiments, at least a part of the plasmonic nano-antennas of the set of plasmonic nano-antennas is resonant in a first observation spectral band and at least a part of the plasmonic nano-antennas of the set. of plasmonic nano-antennas is resonant in a second spectral band of observation; in these examples, the plasmonic nano-antennas may be arranged spatially on the support so as to form a first spatial coding of a first spatial information or image, observable in the first observation spectral band, and a second spatial coding of a second spatial information or image, observable in the second spectral band of observation.

According to one or more exemplary embodiments, the plasmonic nano-antennas are distributed in elementary cells of similar shapes and dimensions, each elementary cell comprising one of said subsets of one or more plasmonic nano-antennas (s). ), the dimensions of an elementary mesh corresponding for example to the dimensions of a pixel.

According to one or more exemplary embodiments, it is possible within the same elementary cell, to combine plasmonic nano-antennas having different resonance wavelengths for the same polarization; an optical response is then obtained in a spectral band covering all the resonant wavelengths resulting from an additive synthesis of the optical responses of each of the nano-antennas in the case of a transmission observation or we obtain a optical response resulting from a subtractive synthesis of the optical responses of each of the nano-antennas in the case of an observation in reflection.

According to one or more exemplary embodiments, it is also possible within the same elementary cell, to combine plasmonic nano-antennas having different resonance wavelengths for orthogonal polarizations. It is thus possible to code a first spatial information according to a first polarization and a second spatial information according to a second polarization.

According to one or more exemplary embodiments, the plasmonic nano-antennas are distributed according to first elementary meshes of similar shapes and dimensions, each of the first elementary meshes having an optical response in a first spectral band of observation, and the first elementary meshes are distributed according to second elemental meshes of similar shapes and sizes, each of the second elementary meshes having an optical response in a second spectral band of observation. This variant notably allows the coding of two spatial information in two different spectral observation bands.

According to one or more exemplary embodiments, the plasmonic nano-antennas are spatially distributed on the substrate to code at least in a first spectral band of observation and according to a given polarization a spatial information, or image, forming a QR code.

According to one or more exemplary embodiments, the plasmonic nano-antennas are spatially distributed on the substrate to code at least in a first observation spectral band and according to a given polarization spatial information, or image, forming a recognizable pattern.

According to one or more exemplary embodiments, the set of plasmonic nano-antennas comprises a first continuous metal layer, a second continuous layer of dielectric material formed on the first metal layer, a third metal layer structured to locally form metal-dielectric overlays. -metal forming the plasmonic nano-antennas. A coding device thus constituted can be produced by means of simple processes and industrially controlled by "nano-imprint" techniques.

By dielectric material is meant any material or combination of materials whose imaginary part of the index does not exceed 0.2 in the spectral band of interest.

According to one or more exemplary embodiments, the coding device further comprises a substrate forming the support and on which is deposited the first continuous layer of metallic material.

Alternatively, the support can be formed directly by the first layer of metallic material. For example, the support can be formed directly by a metal part of an object to be secured.

Alternatively, the support can be formed directly by the second layer of dielectric material, for coding devices of reduced lateral dimensions because of the limited thickness of the second layer of dielectric material (typically less than one-tenth of the wavelength minimum spectral band of observation considered).

According to a second aspect, the present description relates to a secure object provided with a coding device according to any one of the preceding claims. The secure object is for example an object chosen from one of the following categories: jewelry, perfume bottles, clothing, bank cards, bank notes, identity documents (identity cards, passports, driving licenses, etc. .) or any valuable document (gift certificates, etc.).

According to a third aspect, the present description relates to a method of coding a spatial information, or image, in a given spectral observation band, by means of a coding device according to the first aspect.

• la décomposition de l'image en pixels, chaque pixel présentant une position dans l'image ;
• l'attribution à chaque pixel d'au moins une valeur de pixel;
• pour chaque pixel, la détermination d'un sous-ensemble d'une ou plusieurs nano-antenne(s) plasmonique(s) dont la réponse optique selon une polarisation et dans une bande spectrale comprise dans la bande spectrale d'observation correspond à la valeur du pixel ;
• la réalisation des structures métal-diélectrique-métal sur le support pour la formation de l'ensemble des nano-antennes.

According to one or more exemplary embodiments, the coding method comprises:

• decomposing the image into pixels, each pixel having a position in the image;
• assigning each pixel at least one pixel value;
• for each pixel, the determination of a subset of one or more plasmonic nano-antenna (s) whose optical response according to a polarization and in a spectral band included in the spectral band of observation corresponds to the pixel value;
• the realization of the metal-dielectric-metal structures on the support for the formation of all the nano-antennas.

According to one or more exemplary embodiments, the image being decomposed into pixels of identical shapes and dimensions, the coding method comprises for each pixel the determination of an elementary mesh defined by a position on the support, a shape and a dimension corresponding respectively to the position of the pixel in the image, the shape and the size of the pixel and, for each elementary mesh, the determination of said set of nano-antennas whose optical response is equal to the value of the pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

• FIG. 1A et 1B, des schémas illustrant deux exemples de dispositifs de codage selon la présente description;
• FIG. 2A et 2B, respectivement un schéma montrant un exemple d'une maille élémentaire d'un dispositif de codage et une vue en coupe de ladite maille élémentaire ;
• FIG. 2C à 2E des courbes montrant respectivement l'émission et l'absorption normalisées de nano-antennes plasmoniques, pour différentes valeurs de longueurs d'onde de résonance dans la bande 3 - 5 µm ; la luminance du corps noir comparée à des courbes d'émission de nano-antennes plasmoniques, à différentes valeurs de longueurs d'onde de résonance ; un exemple de valeurs en niveaux de gris de la réponse optique en fonction de la largeur de nano-antennes plasmoniques, dans le cas de nano-antennes de forme parallélépipédique rectangle ;
• FIG. 3A à 3H, des schémas illustrant des motifs élémentaires de nano-antennes MDM, adaptés pour la formation d'un dispositif de codage selon la présente description;
• FIG. 4A à 4D, des schémas illustrant des motifs formés de combinaisons d'antennes MDM pour la formation de mailles élémentaires d'un dispositif de codage selon la présente description;
• FIG. 5A et 5B, respectivement un exemple de dispositif de codage selon la présente description formé d'un ensemble de mailles élémentaires comprenant des nano-antennes résonantes selon deux polarisations orthogonales, et les réponses optiques de chacune des mailles élémentaires observées dans la bande 8 - 12 microns, selon chacune des polarisations;
• FIG. 6, un exemple de dispositif de codage selon la présente description formé d'un ensemble de mailles élémentaires comprenant des nano-antennes résonantes selon deux polarisations orthogonales et agencées pour coder un code QR, selon un exemple de réalisation de la présente description;
• FIG. 7A à 7D, les réponses optiques du dispositif de codage codant le code QR illustré sur la figure 6, selon chacune des polarisations et dans deux bandes spectrales;
• FIG. 8A, une première réponse , selon une première polarisation et dans une première bande spectrale, d'un dispositif de codage selon la présente description observé en émission, le dispositif de codage comprenant un ensemble de mailles élémentaires comprenant des nano-antennes résonantes selon deux polarisations orthogonales et agencées pour coder une première image selon une polarisation et une deuxième image selon une deuxième polarisation;
• FIG. 8B, la réponse optique, selon la deuxième polarisation et dans la première bande spectrale, du dispositif de codage dont une première réponse est illustrée sur la figure 8A, observé en émission;
• FIG. 9A à 9E, les réponses optiques, selon la deuxième polarisation et dans des sous-bandes spectrales de la première bande spectrale, du dispositif de codage dont une première réponse est illustrée sur la figure 8A, observé en émission.
• FIG. 10, les étapes d'un procédé de codage selon un exemple de la présente description.
• FIG. 11A et 11B, deux schémas illustrant des dispositifs adaptés pour l'authentification d'un objet sécurisé respectivement en réflexion, avec une source d'éclairage, et en émission, avec des moyens de chauffe.

Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures:

• FIG. 1A and 1B diagrams illustrating two examples of coding devices according to the present description;
• FIG. 2A and 2B respectively a diagram showing an example of an elementary mesh of a coding device and a sectional view of said elementary mesh;
• FIG. 2C to 2E curves respectively showing the normalized emission and absorption of plasmonic nano-antennas, for different resonance wavelength values in the 3 - 5 μm band; the luminance of the black body compared to emission curves of plasmonic nano-antennas at different resonance wavelength values; an example of grayscale values of the optical response as a function of the width of plasmonic nano-antennas, in the case of rectangular parallelepiped-shaped nano-antennas;
• FIG. 3A to 3H diagrams illustrating elementary patterns of MDM nano-antennas adapted for the formation of a coding device according to the present description;
• FIG. 4A to 4D diagrams illustrating patterns formed of combinations of MDM antennas for the formation of elementary cells of a coding device according to the present description;
• FIG. 5A and 5B , respectively an exemplary coding device according to the present description formed of a set of elementary cells comprising resonant nano-antennas according to two orthogonal polarizations, and the optical responses of each of the elementary cells observed in the 8 - 12 micron band, according to each of the polarizations;
• FIG. 6 an exemplary coding device according to the present description formed of a set of elementary cells comprising resonant nano-antennas according to two orthogonal polarizations and arranged to encode a QR code, according to an exemplary embodiment of the present description;
• FIG. 7A to 7D , the optical responses of the coding device encoding the QR code illustrated on the figure 6 according to each of the polarizations and in two spectral bands;
• FIG. 8A , a first response, according to a first polarization and in a first spectral band, of a coding device according to the present description observed in transmission, the coding device comprising a set of elementary cells comprising resonant nano-antennas according to two polarizations orthogonal and arranged to encode a first image according to a polarization and a second image according to a second polarization;
• FIG. 8B , the optical response, according to the second polarization and in the first spectral band, of the coding device, a first response of which is illustrated on the figure 8A , observed in emission;
• FIG. 9A to 9E , the optical responses, according to the second polarization and in the spectral sub-bands of the first spectral band, of the coding device, a first response of which is illustrated on the figure 8A , observed in emission.
• FIG. 10 , the steps of an encoding method according to an example of the present description.
• FIG. 11A and 11B two diagrams illustrating suitable devices for the authentication of a secure object respectively in reflection, with a light source, and in transmission, with heating means.

DETAILED DESCRIPTION

The Figures 1A and 1B illustrate two examples of coding devices according to the present description.

The coding device 10 comprises, in each of these examples, a substrate 11 forming a support, a first continuous metal layer 12 deposited on said substrate, a layer of dielectric material 13 deposited on the metal material layer 12 and a second metal layer structured to form a set of metal "studs" 400 whose geometrical characteristics are chosen so as to form, with the layer 13 of dielectric material, metal-dielectric-metal (MDM) structures each forming a plasmonic nano-antenna and which will be described in more detail later.

The substrate is for example chosen from glass, silicon or plastic.

The dielectric material is chosen from any material or combination of materials whose imaginary part of the index does not exceed 0.2 in the spectral band of interest and which, preferably, does not exhibit absorption. For example, the layer of dielectric material comprises an oxide (eg silica (SiO 2), titanium oxide (TiO 2), magnesium oxide (MgO 2), alumina (Al 2 O 3), zinc sulphide (ZnS), a glass, a material plastic or resin (eg polymethylmethacrylate (PMMA), polyethylene terephthalate (PET), capton, benzocyclobutene (BCB) ...), a textile material (eg silk), or a combination of two or more layers of these materials, including or not a layer of air. These materials cover the spectral range from UV to THz.

The first and second metal layers are for example chosen from gold, aluminum, copper, or silver. Aluminum and silver make it possible to extend the operation of the coding device to the UV range (300-450 nm), and all these materials operate from visible to THz (typically 300 μm).

According to another embodiment (not shown in the figures), the support may be formed by one or other of the metal layer 12 and the dielectric material layer 13, in which case the substrate 11 is not necessary. When it is an object to be secured by means of the coding device according to the present description, the continuous metallic layer 12 may be formed by a metal part of said object and form a support.

As illustrated on the Figures 1A and 1B , the geometric characteristics of the pads 400 (shape, dimensions, orientation) are chosen as a function of their position in the plane of the support for coding a spatial information or "image" formed of a set of pixels defined by a position and one or more values (eg, gray level, color).

More specifically, in the exemplary embodiments illustrated on the Figures 1A and 1B , the pads 400 are grouped in the form of a set of elementary meshes 100 of similar shapes and dimensions, distributed evenly over the substrate. The shapes of the elementary meshes can be various: square, rectangular, triangular, hexagonal, octagonal, etc. The elementary meshes each correspond to a "pixel" of the spatial information that one seeks to encode and their dimensions are therefore adapted to the dimensions of the pixels. In practice, the dimensions of the pixels can be chosen according to the device for authenticating the coding device and more precisely the detector and the image forming lens on the detector. Thus, the minimum dimensions of the pixels will be of the order of half of the maximum wavelength of the observation spectral band, which corresponds to the diffraction limit of the imaging objective. Depending on the size of an elementary cell, there may be one or a subset of nano-antennas per elementary cell, resonant at the same wavelength or at different wavelengths.

In the examples of Figures 1A and 1B each elementary cell comprises a subset of pads 400 forming plasmonic nano-antennas. The pads are rectangular parallelepipeds whose length defines the resonance wavelength. They are oriented in one direction in the example of the Figure 1A , while in the example of the Figure 1B the studs are rectangular parallelepipeds oriented in two perpendicular directions.

In the example of the Figure 1A , the optical response is observed according to a polarization and results from an additive synthesis of the optical responses of each of the nano-antennas in the case of an emission observation or results from a subtractive synthesis of the optical responses of each of the nanoparticles. antennas in the case of an observation in reflection.

The optical response of an elementary cell in the case of the example of the Figure 1B differs depending on whether the TM or TE polarization coding device is observed and results, as in the example of FIG. Figure 1A of an additive or subtractive synthesis of the responses of each nano-antenna of the elementary cell, according to one or other of the polarizations. A coding device as shown in the Figure 1B thus allows the coding of two spatial information, a first spatial information according to a first polarization and a second spatial information according to a second polarization. In practice, the observation of a coding device as represented on the Figure 1B and coding the first and second spatial information can be done in reflection or in transmission through a polarizer whose axis is oriented according to one or the other of the polarizations.

The Figures 2A and 2B respectively illustrate a diagram of an elementary cell 101 of a coding device as illustrated in FIGS. Figures 1A or 1B and a sectional view of this elementary mesh.

In this example, the elementary mesh is square, and has a side of dimension A. On the elementary mesh 101 are arranged two metal studs 401, 402 of parallelepipedal shape, of respective dimensions (L ₁ , w ₁ ), (L ₂ , w ₂ ), separated by a distance d, oriented in the same direction which defines the polarization of the absorbed (or emitted) wave. The Figure 2B represents a sectional view at the level of the stud 401.

The height of the studs is advantageously greater than the skin thickness of the metal (typically 25 nm for gold 600 nm to 12 microns) to prevent leakage of the MDM cavity via the upper metal layer.

It is known that a metal-dielectric-metal structure thus formed at each of the metal pads 401, 402 forms a nano-plasmonic antenna having a resonance wavelength (respectively λ _R1 , λ _R2 ) depending on the length the nano-antenna (respectively L ₁ , L ₂ ) and the choice of the dielectric material. It is shown that in first approximation (see for example Cui et al., Laser & Photonics Review pp 500 - 502 (2014 )), the length L _i of a rectangular nano-antenna sets the resonance wavelength λ _Ri to within 10% according to equation (1) below: $${\mathrm{\lambda }}_{\mathrm{Ri}}\approx 2{\mathrm{not}}_{\mathrm{D}}{\mathrm{The}}_{\mathrm{i}}$$

Where n _D is the refractive index of the dielectric material.

Such a nano-antenna has an extraordinary absorption of an incident wave at said wavelength and for a so-called transverse polarization (TE) of the incident light wave, that is to say for the component of the wave incident light whose magnetic field H is perpendicular to the direction of the nano-antenna according to which the length L is measured; (see Figure 2A ). Such a nano-antenna also has an extraordinary emission at said wavelength of a light wave having a transverse polarization, the emission response (or luminance) of the nano-antenna at a given temperature T being the product of the resonant emission (emissivity) with the emission of the black body at temperature T (luminance of the black body).

The Figure 2C thus illustrates curves showing the absorption (dashed) and the emission (solid line) of plasmonic nano-antennas having different resonant wavelengths.

The 2D figure represents the luminance curve of the blackbody at 373K and plasmonic nano-antennas of MDM type optimized to absorb 100% of the flux (emissivity of 1) at different resonant wavelengths (4, 4.5, 5, 5, 5 and 6 μm).

The quality factor Q of such a nano-antenna, equal to the ratio between the resonance wavelength λ _Ri and the half-width of the resonance (FWHM), once the chosen materials, depends in a known manner several geometrical parameters such as the thickness e of the layer of dielectric material, the width l and the length L of the nano-antenna. By working with a given thickness e of the layer of dielectric material, between λ _Ri / 100 and λ _Ri / 5, it is thus possible to vary the optical response in reflection or in emission of a nano-antenna to a length of given observation wave, from 0% to 100% and according to a given polarization, playing on the width, the length or the orientation. The determination of these parameters to obtain a given response can be done using known calculation software, such as Comsol © software.

The spectral range of operation of a nano-antenna therefore depends only on the choice of the dielectric material and can range from UV (from 350 nm) to THz (up to 300 μm).

In the example shown on the Figure 2A the choice of two metal pads arranged in the same direction but of different lengths allows resonance of the nano-antennas thus formed at two different resonance wavelengths, for the same polarization. At the level of the mesh 101, this results in an optical response which is an additive or subtractive synthesis of the responses of each of the nano-antennas, depending on whether the emission or reflection coding device is observed, the responses of the two nano-antennas. antennas formed by the studs 401, 402.

Thus, by means of nano-antennas of rectangular parallelepipedal shape as described on the Figures 2A and 2B , or in the examples of Figures 1A and 1B , it is possible to carry out an encoding of one or more spatial information, in color or in gray level, for an observation in reflection or in emission, according to a given polarization and / or at a given wavelength or in a given range of observation. The following paragraphs give examples of grayscale or color coding using rectangular nano-antennas.

Examples of coding spatial information in gray level:

We start from a grayscale image, for example a two-dimensional pixelated image. Each pixel is therefore associated with a pixel value, which is a gray level.

According to a first variant, it is sought to reproduce this level of gray in reflection or emission in a spectral measurement band centered on a wavelength λ included in the spectral band of observation.

According to a first variant, the measurement spectral band has a width typically of λ / 10, and corresponding to the width of the response of a resonant nano-antenna at the wavelength of observation (see FIG. Figure 2C ). In practice, the measurement can be done through an appropriate filter.

In order to obtain the gray level, according to this example, a nano-antenna of metal-dielectric-metal type of rectangular shape, as previously described, is used. The length L of the nano-antenna sets the wavelength of the maximum absorption or thermal emission (see equation 1).

By properly choosing the thickness of the dielectric (which is then a fixed parameter for all the nano-antennas), it is thus possible according to a first example to obtain all absorption or emission levels from "0%" to "100%" by changing the width of the nano-antenna, resulting in known manner a change in the efficiency of the nano-antenna compared to a calculated width to optimize the response. For example in absorption, the "0%" level may in practice correspond at best to the residual absorption of the metal, typically of the order of 5% in the visible, 2 - 3% in the infrared and 1% in the THz, but with a variability that depends on the nature of the metal.

The figure 2E thus shows optical responses measured in reflection at 4.2 μm as a function of the width w of plasmonic nano-antennas for a 1.2 μm long nano-antenna, a silica dielectric material (thickness 220 nm, index 1.4 ). In this example, the metal is gold and the continuous metal layer has a uniform thickness greater than 100 nm and therefore optically opaque. It is observed in this example that for a width of the nano-antenna of about 100 nm, the reflection is minimal (the absorption is maximum); by varying the width, one increases the reflection, this increase resulting from a less good absorption by the nano-antenna.

The pixel can be encoded on an elementary cell whose minimum dimensions are λ / 2, corresponding to the diffraction limit below which far-field structuring can not be attributed. If one tries to have a pixel of side bigger than λ, one can put several nano-antennas by mesh elementary. For example, if we look for a pixel side ~N ^* λ, then we can repeat periodically the nano-antenna, for example with a period in the 2 directions of ~λ / 2.

According to a second example, and in the case of pixels of very large dimensions in front of λ, the gray level can be obtained by virtue of the density of nano-antennas only.

According to a third example, it is also possible to obtain the gray level in the given spectral band of width equal to typically λ / 10, by increasing / decreasing the length of the nano-antenna. Indeed, the response of the antenna is typically Lorentzian (see Figure 2C ); thus, when the length of the antenna is changed, the wavelength of resonance is shifted wavelength and therefore the response is lower in the spectral band of observation.

In the case of operation in emission of the nano-antennas ( 2D figure ), a gray-level coding can also be obtained by modifying the length of the bar forming the plasmonic nano-antenna, which results in a displacement of the emission curve of the nano-antenna on the curve of the black body and of this makes a change in the optical response.

According to a fourth example, it is possible to obtain the gray level by varying the orientation of the rectangular antenna in the plane. If the optimized antenna is along the u axis, we will have the gray level of cos (φ) ² for the same rotated antenna of φ.

According to a second variant, the spectral band for measuring the optical response may be wider. In this case, a gray level can be obtained by combining several nano-antennas, for example several nano-antennas of different lengths each having a resonance wavelength in the observation spectral band. The gray level can then be obtained by adjusting the response of each nano-antenna according to one of the examples described above (width of the antenna, orientation, density), or by the absence of certain antennas.

According to one or other of the variants / examples described, it is understood that it is possible to code several independent spatial information. For example, several pieces of information can be encoded in several spectral observation bands. The choice of a suitable filter will allow the observation of a given information. It is possible to code independent spatial information according to the two orthogonal polarizations. Choosing a suitable polarizer during the observation will allow the observation of a given piece of information.

Examples of encoding color spatial information:

In this example, we start with a color spatial information, for example a pixelated, two-dimensional image. For each pixel of the image, one can for example define a level "RGB" (red green blue), which gives a color among a number of possible colors identified, for example 16 million possible colors.

According to one example, for each of the three colors, it is possible to define a gray-scale image and to encode it according to the previously described method of grayscale encoding. Thus, for a given pixel, three resonant nano-antennas with three increasing wavelengths λ _r1 , λ _r2 and λ _r3 are chosen. The observation can be made with the naked eye, by means of a multi spectral camera with RGB filters on each pixel or through three filters respectively. These three wavelengths are advantageously spectrally separated by at least λ _r3 / 10. The gray level of each antenna can be determined for example by the width of the antenna or its orientation (polarized response only). It is also possible to adjust the gray level of each antenna by playing on its length if the spectral separation between the 3 resonant wavelengths is large, ie by modifying the length to obtain a lower level of absorption, we do not create a signal in the ^2nd spectral band of interest. This results in practice by a spectral separation of at least λ _r3 / 5.

As before, the pixel can be encoded on an elementary cell whose minimum dimensions are of λ _r3 / 2, corresponding to the diffraction limit below which far-field structuring can not be attributed.

It is thus possible to encode several color information in different spectral bands, for example an image in the visible and an image in the infrared. In practice, the 2 encoding pixels have dimensions that are multiples of one another (for example, a visible encoding grid with pixels of 250 nm and an infrared grid with pixels of 2 μm), each infrared pixel containing 64 visible pixels.

It is also possible to encode one or more information according to one or the other of the polarizations.

It is also possible to encode one or more greyscale information and one or more color information, in one or more spectral bands, according to one or other of the polarizations.

Although a particular embodiment has just been described using nano-antennas formed by means of rectangular parallelepiped shaped pads, it is known to those skilled in the art that nano-antennas of different shapes can be realized. and that the shape of the metal pads for the formation of the plasmonic nano-antennas is not limited to rectangular parallelepipeds.

Thus, Figures 3A to 3H represent according to views from above a set of metal studs adapted for the formation of metal-dielectric-metal structures forming plasmonic nano-antennas. The structures having a top view of square-type shapes ( FIG. 3A ) circle ( FIG. 3B ), cross ( FIG. 3C ) and combination of rectangles of the same length and along two perpendicular axes ( FIG. 3D ) have polarization-insensitive responses Structures with rectangular shapes in top view ( FIG. 3E ), ellipse ( FIG. 3F ), asymmetric cross ( FIG. 3G ) and combination of several rectangles of different lengths along perpendicular axes ( FIG. 3H ) have an optical response that depends on the polarization.

The Figures 3E, 3D and 3H present combinations of rectangular parallelepiped shaped pads already described by means of Figures 2A to 2D . The optical response may vary in "color" or gray level as previously described. The Figures 3C and 3G have plasmonic nano-antennas which in top view have cross shapes. These structures have behaviors that are substantially similar to the behaviors respectively of the nano-antennas represented on the 3D figures (rectangles of the same length) and 3G (rectangles of different lengths) and have dimensioning rules described for example in Cui et al., Laser & Photonics Review 8, 495 (2014) ).

Plasmonic nano-antennas obtained by means of square metal studs (see figure 3A ) have, for example, been described in Cui et al., Laser & Photonics Review 8, 495 (2014) ). The sizing rules are similar to those of nano-antennas of rectangular parallelepiped shape but they have an optical response independent of the polarization. In the case of square-shaped nano-antennas, gray level coding can be obtained by observing in a given spectral band of observation and by varying the size of the square, as has been described previously.

Plasmonic nano-antennas having a circular shape in a top view are for example described in the same review article. Cui et al., Laser & Photonics Review 8, 495 (2014) ). Again, the optical response is polarization independent and the sizing rules are substantially similar to those of square-shaped plasmonic nano-antennas. In the same way as for square-shaped nano-antennas, gray-level coding can be obtained by observing in a given spectral band of observation and varying the diameter of the circle.

Plasmonic nano-antennas having an elliptical shape in plan view have dimensioning rules substantially similar to those of the rectangular-shaped plasmonic nano-antennas.

Other patterns (star, triangle, cross more complex, etc.) are also possible but have the disadvantage of less flexibility to design the optical response of the mesh.

The Figures 4A to 4D illustrate, also in plan views, elementary meshes in which are associated plasmonic nano-antennas having different geometric shapes.

Each nano-antenna having a resonance length of its own, an elementary cell thus conceived makes it possible to generate an optical response that results from an additive or subtractive synthesis, depending on whether the transmission or reflection coding device is being observed. different nano-antennas. In the case of spectral coding of the information, this allows access to a greater number of optical responses or "colors". Here again in this example, some of the elementary meshes have insensitive responses to polarization ( FIG. 4A, FIG. 4B, 4C ) while the FIG. 4D illustrates an example in which "color" depends on polarization.

The Figures 5 to 9 illustrate in more detail exemplary embodiments of the coding device according to the present description. They implement nano-antennas form rectangular but could equally well be designed with nano-antennas having different shapes such as those described above, the choice depending in particular the desire to have nano-antennas are the optical responses are polarized or not.

The Figures 5A and 5B illustrate an example of application of a coding device 10 according to the present description, allowing a first spectral coding according to a first polarization and a second spectral coding according to a second polarization, the device being intended to be observed in reflection in a spectral band given.

The Figure 5A represents the coding device 10 seen from above; only the shapes of the elementary meshes and the plasmonic nano-antennas arranged in each of the meshes are represented. The coding device is composed of 12 identical elementary cells 101 - 112 of square shape, the size of which is adapted to the size of the pixel of the information that is to be encoded. Within each elementary cell are associated plasmonic nano-antennas with responses according to each of the two polarizations. The horizontal nano-antennas (according to x) encode the TM polarization and the vertical antennas (according to y) encode the TE polarization. In this example, each plasmonic nano-antenna has a rectangular shape arranged in one or the other of two perpendicular directions. In each elementary cell, it can have up to 4 nano-plasmonic antennas of different lengths in one direction, which therefore have in the spectral band of observation 4 different resonance wavelengths.

The presence or absence of each of the nano-antennas of different lengths makes it possible, according to each polarization, to form 2 ⁴ = 16 different optical responses in the spectral band of observation, when the coding device is observed in reflection or in emission. The Figure 5B thus illustrates the optical responses 301 - 312 as a function of the wavelength calculated for each elementary cell according to each of the TE and TM polarizations (the spectra are represented respectively in full line (TE) and dotted line (TM)).

The figure 6 represents an exemplary encoding device 10 applied to the realization of a QR-type code (abbreviation of "Quick Response") or two-dimensional barcode type code, in color, observable in two spectral bands (visible and infrared ) and according to two polarizations (TE, TM). The Figures 7A, 7B and 8A, 8B represent the optical responses in each of the spectral bands.

The figure 6 comprises a first set of elementary meshes 100 and a second set of elemental meshes 200, the elementary meshes 200 each comprising a subset of elementary meshes 100.

The elementary meshes 100 are sized to form an optical response in the visible, according to two orthogonal polarizations. Thus, each elementary cell comprises a first set of nano-antennas oriented in a first direction (for example parallelepiped-shaped nano-antennas) for coding a first information according to a first polarization and a second set of oriented nano-antennas. in a direction perpendicular to the coding of a second information according to a second polarization. More precisely, the nano-antennas represented horizontally (according to x) on the figure 6 encode the "vertical" polarization or TM and the nano-antennas represented vertically (according to y) on the figure 6 encode for "horizontal" (or TE) polarization. In this example, the coding is a "color coding" as previously described. Thus, each nano-antenna can take one of three lengths allowing resonant absorption at one of the three resonance wavelengths λ _r1 , λ _r2 and λ _r3 located respectively in blue, green and red. In this example, each nano-antenna is either present or absent, which results in 8 possible combinations to form 8 colors, namely red, dark blue, green, white, black, pink, light blue and yellow, as this is illustrated on the Figures 7A and 7B . The observation can be done with the naked eye, but also by all cameras / cameras.

The elementary meshes 200 are sized to form an optical response in the infrared (around 2 - 3 μm), also according to two orthogonal polarizations. In this example, each elementary cell 200 comprises a first set of at most 2 nano-antennas oriented in a first direction for the coding of a first information in the infrared according to a first polarization and a second set of at most 2 nano-antennas oriented in a perpendicular direction for the coding of a second information in the infrared according to a second polarization. The coding is also in this example a "color coding" as previously described. Each nano-antenna may take one of two lengths allowing resonant absorption at one of the two resonance wavelengths λ _r4 , λ _r5 located respectively in the band 2 - 3 μm. In this example, each nano-antenna is either present or absent, which results in 4 possible combinations making it possible to form 4 colors as illustrated on the Figures 7C and 7D . Observation can be done through a polarizer, using a standard infrared camera.

The Figures 8A and 8B , 9A to 9E illustrate the transmission observation of a coding device according to the present description, produced by means of rectangular parallelepiped-shaped plasmonic nano-antennas arranged on elementary meshes of dimensions 30 × 30 microns. In this example, the substrate is silicon, the metal is gold, the dielectric formed of silica. The lower metal layer has a thickness of 200 nm (optically opaque layer). The thickness of the dielectric layer is 220 nm. The metal pads have a thickness of 50 nm. The antennas have widths of 100 nm and their lengths in one of the directions vary between 900 and 1450 nm, in steps of 50 nm to encode 11 levels of gray emission in one polarization and have 5 different lengths in the other direction . The temperature of the sample is 373 ° C for an observable emission in the spectral band 3 - 5 microns. In this example, a first "Molière" image ( figure 8A ) is coded according to a first polarization and a second image, formed of a superposition of the letters "M", "I", "N", "A", "O" is coded according to a second polarization. Thus, Figures 8A and 8B result from the emission observation of the heated device through two crossed polarizers. In these examples, the coding of the "Molière" is done in gray level, by modifying the length of the bar forming the plasmonic nano-antenna, which results in a displacement of the emission curve of the nano-antenna on the curve. black body (see 2D figure ) and thus a change in the optical response. The coding of all the letters is done in grayscale in 5 different spectral bands, centered respectively 3.20 microns, 3.71 microns, 4.22 microns, 4.73 microns, 5.24 microns. The gray level is obtained for each letter by changing the length of each antenna. The Figures 9A to 9E thus show the observation through filters respectively centered on each of the wavelengths.

For all these examples, a coding method as represented on the figure 10 can be used.

The figure 10 illustrates an exemplary method of encoding at least one spatial information or "image" by means of an encoding device according to the present description, for example a coding device comprising plasmonic nano-antennas as described above. Spatial information is, for example, spatial information forming a recognizable pattern (for example the "Molière" or the letters of Figures 8A and 8B ), or spatial information forming a QR-type bar code as described in average of figures 6 and 7A to 7D , or can be an image representative of one-dimensional spatial information, for example a one-dimensional bar code. In all cases, it is sought to adapt the coding of the information to the authentication devices and in particular to the parameters of the detector (spectral detection band, pixel size) and the focusing optics (numerical aperture of the objective ), for example as described in the Figures 11A and 11B .

The image (s) are firstly cut into pixels or "pixelated" (step S1), the pixel size depending on the parameters of the detection systems. For each pixel P _{j, i} of each image, where i is the position of the pixel in the index image j, a value is assigned which can be, as previously described, a gray level in a range of given observation, or a "color", that is to say a set of several values of gray levels for different wavelengths or ranges of wavelengths (step S ₂ ). Then (step S ₃ ) the elementary mesh at position i is determined on the support of the coding device which will make it possible to form the optical response (s) of given value for each pixel, according to the encoding methods. previously described. The last step (S4) then consists in manufacturing the coding device, according to known manufacturing methods, for example metallic deposition on a substrate, deposition of the dielectric layer, electronic lithography (but which can be replaced by UV or nanoimprint lithography) for the formation of metal pads, followed by a lift off (see for example Levesque et al. "Plasmonic planar antenna for wideband and efficient linear polarization conversion", Appl. Phys. Lett 104, 111105 (2014) )).

The Figures 11A and 11B are two examples of secure product authentication devices by means of encoding device according to the present description, for an authentication respectively in reflection and in transmission.

The authentication device represented on the figure 11A is adapted to authentication in reflection of a coding device 10 according to the present description. The coding device 10 is for example integrated in an object to be secured (not shown). The authentication device comprises a transmission channel with a source of emission 20 for the emission of a collimated light beam I intended to illuminate the coding device 10. The emission source comprises, for example, a transmitter 21 and a transmitter. optical collimation lens 22. The transmitter is adapted to the desired spectral observation band. For example, the emitter is a visible light source or a light source in the infrared, for example one of the spectral bands 3 - 5 μm or 8 -12 μm corresponding to atmospheric transmission bands. The authentication device further comprises a detection channel with a detection system 30 for receiving a beam R resulting from the reflection of the illumination beam I by the coding device. The detection system 30 comprises an optical focusing element 31 which can be formed for example of an objective, an optical lens or any combination of these elements, and a detector 32 for the detection in the spectral observation band . The detector comprises for example a CCD or CMOS camera for observation in the visible, with pixel sizes of 1 to 10 μm. In the infrared, the detectors may comprise, for example: microbolometer detectors (3 - 14 μm), MCT detectors I (1.5 - 5 μm), InGaAs detectors (1 - 1.8 μm). The detection path further comprises, in one or more embodiments, one or more polarizers 50 and one or more spectral filters 40. The detection system defines a "pixel size" limited by the opening of the optics of focusing 31 or the size of an elementary detector of the detector 32. The pixel size is typically 1 to 10 μm in a visible detection system; it is limited by the diffraction limit, with an influence on the signal-to-noise ratio which decreases when the pixels become small. The pixel size is typically 15 μm in an infrared detection system but should drop to 10 μm for next-generation detectors.

The authentication device represented on the Figure 11B is adapted to transmit authentication of a coding device 10 according to the present description. It comprises a detection path substantially similar to that shown on the figure 11A but no transmission path since it is the thermal emission of the coding device which is measured and not the reflection of an incident optical wave. According to one or more exemplary embodiments, the authentication device comprises heating means 60 enabling thermal emission at wavelengths in conventional infrared detection bands.

In the visible, the coding devices can also be authenticated with the naked eye. Thus, with normal vision, an individual can distinguish patterns with an angular resolution of 1 minute arc, which corresponds for an object observed at the punctum proximum (typically at a distance of 25 cms) to see pixels on the object that are between 7 and 8 μm. It is possible to go down to the visible diffraction limit with conventional devices (magnifying glass, microscope).

Although described through a number of detailed exemplary embodiments, the device and the coding method according to the present description comprise different variants, modifications and improvements which will become apparent to those skilled in the art, it being understood that these various variants, modifications and improvements are within the scope of the invention, as defined by the following claims.

Claims (14)

1. An optical encoding device (10, 20, 30, 40) of an image formed of pixels each defined by a position and at least one value, the image comprising at least two pixels of different values, the encoding device being intended to be observed in at least a first spectral band of observation and comprising:

   - a supporting structure;

   - a set of plasmonic nanoantennas of the metal-dielectric-metal type formed on said supporting structure, such as:

   - each plasmonic nanoantenna is resonant to at least one wavelength comprised in said first spectral band of observation,

   - the plasmonic nanoantennas are arranged spatially on the supporting structure so that at one pixel of the image, a subset of one or more plasmonic nanoantenna(s) is associated, whose optical response according to a polarization and in a spectral band comprised in the first spectral band of observation corresponds to the value of said pixel, the set of plasmonic nanoantennas thus forming at least a first spatial coding of said image in said first spectral band of observation.
2. The encoding device according to claim 1, wherein at least a first spatial encoding comprises a grayscale coding, said optical response of a subset of one or more plasmonic nanoantenna(s) associated with a pixel having a relative intensity in a scale of intensities corresponding to a pixel value defined by a gray level in a scale of gray levels.
3. The encoding device according to any of the preceding claims, wherein a pixel of the image having at least a first and a second value, a subset of nanoantennas is associated with each pixel of the image, wherein said subset of nanoantennas has, according to a given polarization, at least a first and a second optical responses respectively in a first and second spectral bands comprised in the first spectral band of observation, the first and second optical responses corresponding to the first and second values of the pixel, thus forming a spatial color coding of the image in said first spectral band of observation.
4. The encoding device according to any of the preceding claims, wherein:

   - the set of plasmonic nanoantennas comprises resonant plasmonic nanoantennas in a first spectral band of observation and resonant plasmonic nanoantennas in a second spectral band;

   - the resonant plasmonic nanoantennas in the first spectral band are arranged spatially on the substrate so as to form a first spatial encoding of a first image, observable in the first spectral band of observation, and the resonant plasmonic nanoantennas in the second spectral band are arranged spatially on the substrate so as to form a second spatial encoding of a second image, observable in the second spectral band of observation.
5. The encoding device according to any of the preceding claims, wherein:

   - the set of plasmonic nanoantennas comprises resonant plasmonic nanoantennas according to a first polarization and resonant plasmonic nanoantennas according to a second polarization;

   - the resonant plasmonic nanoantennas according to the first polarization are arranged spatially on the substrate in such a way as to form a first spatial encoding of a first image, observable in said first spectral band of observation according to the first polarization, and the resonant plasmonic nanoantennas according to the second polarization are arranged spatially on the substrate so as to form a second spatial encoding of a second image, observable in said first spectral band of observation according to the second polarization.
6. The encoding device according to any of the preceding claims, wherein the plasmonic nanoantennas are distributed in elementary cells (101, 201, 301) of similar shapes and dimensions, each elementary cell comprising a subset of one or more plasmonic nanoantennas having an optical response corresponding to a value of one pixel of the image.
7. The encoding device according to claim 6, wherein the plasmonic antennas are distributed according to first elementary cells (100) of similar shapes and dimensions, each of the first elementary cells (100) comprising a subset of one or more plasmonic nanoantenna(s) presenting an optical response in a first spectral band of observation corresponding to a value of one pixel of a first image, and the first elementary cells are distributed in second elementary cells (200) of similar shapes and dimensions, each of the second elemental cells (200) comprising a subset of one or more plasmonic nanoantenna(s) having an optical response in a second spectral band of observation corresponding to a value of one pixel of a second image.
8. The encoding device according to any of the preceding claims, wherein the plasmonic nanoantennas are spatially distributed on the substrate to encode in a first spectral band of observation and according to a given polarization, at least a first image forming a QR code.
9. The encoding device according to any of the preceding claims, wherein the plasmonic nanoantennas are spatially arranged on the substrate to encode in a first spectral band of observation and according to a given polarization, at least a first image forming a recognizable pattern.
10. The encoding device according to any of the preceding claims, wherein all the plasmonic nanoantennas comprise:

    - a first continuous metal layer (12);

    - a second continuous layer of dielectric material (13) formed on the first continuous metal layer (12);

    - A third metal layer arranged on the second continuous dielectric material layer (13) and structured to locally form metal-dielectric-metal stacks (MDM_i) forming said plasmonic nanoantennas.
11. The encoding device according to claim 10, further comprising a substrate (11) forming the supporting structure on which is deposited the first continuous metal layer (12).
12. The encoding device according to claim 10, wherein the supporting structure is formed by the first continuous metal layer or by the second layer of dielectric material (13).
13. A secured object comprising an encoding device according to any one of the preceding claims.
14. An encoding method in at least a first spectral band of observation of at least one image, by means of an encoding device according to any one of claims 1 to 12, comprising:

    - decomposition of the image into pixels, each pixel having a position in the image;

    - each pixel is assigned the value of at least one pixel;

    - for each pixel, the determination of a subset of one or more plasmonic nanoantenna(s) whose optical response according to a polarization and in a spectral band comprised in the spectral band of observation corresponds to the value of the pixel;

    - the production of the metal-dielectric-metal structures on the supporting structure to form all the nanoantennas.

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